Looking for a small flowering plant that likes a lot of water

One of the posts of the gate that leads to the garden was designed with a planter on top. It’s empty right now.

casaveneracion.com Empty planter

A similar planter is on the post of the second floor veranda.

casaveneracion.com Empty planter on the veranda

Of course, I don’t want them to stay empty. The thing is, these planters have no drainage. So, I can only plant them with something that likes a lot of water all the time.

casaveneracion.com Planter with overgrown fortune plant

The logical choice is the fortune plant. A third planter (near the front door) has a fortune plant that was already there when we moved in and which is now overgrown in every direction. I like fortune plants. But for these planters, I prefer a small flowering shrub that won’t die without well-drained soil. Mums are out. Zinnias too. I’m running out of ideas, to be honest. Suggestions are welcome.